Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
DHW Demand
DHW Selector in Maximum (+) position
CH Selector in Minimum (-) position
ON/OFF Switch in ON position
Gas Off
Remove the spark igniter cap and wedge in place below the igniter against the heat exchanger as shown
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Measurement:

Stand clear of the spark igniter and watch for sparking from the igniter lead to the heat exchanger case.
Note:
Ensure the electrode lead is placed close to the heat exchanger casing for sparking to occur.